A detailed understanding of DNA strand breaks induced by low energy electrons (LEE) is of crucial importance for the advancement of many areas of molecular biology and medicine. To elucidate the mechanism of DNA strand breaks by LEEs, theoretical investigations of the electron attachment-induced C3'-O3' sigma-bond breaking of the pyrimidine nucleotides have been performed. Calculations of 2'-deoxycytidine-3'-monophosphate and 2'-deoxythymidine-3'-monophosphate in their protonated form (denoted as 3'-dCMPH and 3'-dTMPH) have been carried out with the reliably calibrated B3LYP/DZP++ theoretical approach. Our results demonstrate that the transfer of the negative charge from the pi*-orbital of the radical anion of pyrimidines to the DNA backbone does not pass through the N1-glycosidic bond. Instead, the migration of the excessive negative charge through the atomic orbital overlap between the C6 of pyrimidine and the C3' of ribose most likely represents a pathway that subsequently leads to the strand breaks. The proposed mechanism of the LEE-induced single strand breaks in DNA assumes that the formation of the base-centered radical anions is the first step in this process. Subsequently, these electronically stable radical anions may undergo either C-O bond breaking or N-glycosidic bond rupture. The present investigation of 3'-dCMPH and 3'-dTMPH yields an energy barrier of 6.2-7.1 kcal/mol for the C3'-O3' sigma-bond cleavage. This is much lower than the energy barriers required for the C5'-O5' sigma-bond and the N1-glycosidic bond break. Therefore, we conclude that the C3'-O3' sigma-bond rupture dominates the LEE-induced single strand breaks of DNA.